There are generally two types of aperture antennas. The first type of aperture antenna is a horn antenna that is typically included with a cluster or array for directly transmitting and/or receiving radio frequency (RF) signals. The second type of aperture antenna is a reflector antenna, which generally includes a parabolic reflector complemented by one or more feed horns for transmitting and/or receiving RF signals.
It is beneficial that the beamwidth of an aperture antenna, especially in space applications, be as uniform as possible over its operating frequency range, so that the desired radiation pattern produced by the antenna does not substantially vary. A reflector antenna may be modified to produce a constant beamwidth over its operating range by under-illuminating the reflector surface at the higher operating frequencies. The beamwidth of such a modified reflector antenna will be inherently frequency independent due to the self-compensating relationship between the parabolic reflector and feed horn(s), resulting in a substantially uniform beamwidth over its operating frequency range. That is, the significantly oversized reflector surface is fed with a smaller aperture antenna feed. As the beamwidth of the feed antenna decreases with frequency, the illuminated portion of the reflector surface also decrease, causing the effective aperture of the combination to be reduced. This provides an electrical aperture size that is constant with frequency (providing a constant beamwidth). However, under-illuminating the reflector surface results in reflector that is much larger than necessary for the application, which has several disadvantages (increased size, weight, and complexity). Other solutions for providing a constant beamwidth with frequency involve modifications to the reflector surface (either through variable size holes or by using a mesh with variable spacings) to provide reflectivity variations with frequency.
In contrast to this modified reflector antenna, the beamwidth of a horn antenna is frequency-dependent. That is, the beamwidth of a horn antenna is inversely proportional to the electrical aperture size in wavelengths (i.e., larger electrical aperture size translates to smaller beamwidth). For a horn antenna with a fixed physical aperture size, the electrical size in wavelengths increases as the wavelength decreases (i.e., as the frequency is increased). That is, as the frequency of the RF signals increases, the beamwidth decreases, and as the frequency of the RF signals decreases, the beamwidth increases.
While a reflector antenna may be modified to exhibit uniform beamwidth over its operational frequency band, it requires the use of bulky, heavy, and over-sized reflector structures, and therefore may be unsuitable for space applications, suffers from thermal distortion due to the wide variances in temperature in space, and requires a relatively complex manufacturing process. In contrast, a horn antenna is relatively compact and light-weight, is structurally stable, does not suffer from thermal effects, and requires only simple construction and adjustment. However, as can be appreciated from the discussion above, a conventional horn antenna has a beamwidth that is frequency dependent, and due to its broad bandwidth, can exhibit extreme variations in beamwidth over its operational frequency band.
There, thus, remains a need for a constant beamwidth, broad-band, high-gain antenna.